Overview of leachate characteristics for fertigation
The total N content of the raw leachate sample was 900 mg/L (Table 1).
The high N concentration can be attributed to the breakdown of nitrogenous substances during organic waste decomposition.38 The Amper Tenang landfill leachate exhibited typical characteristics of an ageing methanogenic landfill with a BOD5/COD ratio between 0.06 and 0.08 and pH within the range of 6.12–7.04. Christensen et al.39 described this as a characteristic of an ageing landfill. The high suspended solid level in the leachate may be attributed to the presence of organic and inorganic compounds.17
After leachate treatment with 4 g/L FeCl3 at pH 7, the contents of Cd, Al, Fe, Pb, Cu and Zn decreased by 100%, 64.4%, 51.9%, 82%, 56.8% and 96.6%, respectively. The optimal removal capacity for suspended solids (SS) was 80%. Hamidi et al.40 reported the same dosages with similar reduction effects on colour, turbidity, and SS.
Plant physical growth evaluation
B. rapa L. survived until harvest, although common symptoms of soil salinisation such as chlorosis and leaf burn were noticed on 100%RL to 100%TL. This finding was not observed in plants irrigated with 75%DTL–12.5%DTL, probably because of the changes and/
or decrease in concentration gradient. Plants receiving 25%DTL produced significantly longer leaves (23.17±0.577 cm) than those in the other treatments (p<0.05) (Table 4). Plants treated with 25%DTL presented wider leaves, with 1.36 and 3.23 times higher width than plants receiving inorganic fertiliser [100% IF (N15:P15:K15)] and the control. Less expanded leaf length and width were also observed in plants treated with the control compared with those in other treatments, which may be a clear indication of nutrient deficiency and differential N proportions.41,37 Overall, 25%DTL could be the optimal nutrient requirement level for leaf expansion of B. rapa L. (Table 5), at a specific growth rate of 0.53 mm/day.
Table 4: Comparison of leaf length, leaf width, stem height and total leaf number for Brassica rapa L. after 56 days
Treatments Leaf length (cm) Leaf width (cm) Stem height (cm) Total leaf number*(not in cm)
100%RL 19.17±0.577zcfg 9.00±0.866z 2.44±0.323z 16.17±1.893zdeh
100%TL 19.50±1.323zcfgh 9.83±1.155zh 2.23±0.322x 18.67±1.610zcdefh
75%DTL 15.72±1.114z 8.33±1.258z 2.43±0.416z 14.33±0.764z
50%DTL 19.00±1.803zcf 9.00±0.500z 2.27±0.306x 12.00±1.323z
25%DTL 23.17±0.577zybcdfgh 10.33±0.289zch 2.30±0.361x 12.33±1.041z
12.5%DTL 15.67±1.041z 9.33±1.443z 2.17±0.379x 15.17±2.363zde
50%DTL+50%IF 16.83±2.021z 9.17±1.155z 2.50±0.866x 16.33±1.607zdeh
100%IF 17.00±1.500z 7.67±0.764z 2.50±0.866x 13.23±1.662z
dH2O 7.17±0.577 4.50±1.803 1.50±0.500 8.33±0.764
TOTAL 153.23±16.533 77.16±18.466 20.33±4.339 126.56±13.027
Levels of significance: p<0.05 at F=35.256 p<0.05 at F=7.035 p<0.05 at F=1.03 p<0.05 at F=11.651
DTL, diluted treated leachate; TL, treated leachate; IF, inorganic fertiliser; RL, raw leachate
Letters indicate statistical significance between different treatment levels using analysis of variance (ANOVA) version SPSS 17.0.
Table 5: Specific growth rate comparison for Brassica rapa L. at harvest after 56 days
Treatments Leaf length (mm/day) Leaf width (mm/day) Stem height (mm/day) Root length (mm/day-day)
100%RL 0.47 0.35 0.22 0.30
100%TL 0.48 0.38 0.22 0.42
75%DTL 0.42 0.33 0.22 0.40
50%DTL 0.46 0.35 0.20 0.38
25%DTL 0.53 0.39 0.21 0.30
12.5%DTL 0.42 0.37 0.19 0.40
50%DTL+50%IF 0.43 0.36 0.23 0.35
100%IF 0.44 0.31 0.23 0.42
dH2O 0.20 0.16 0.09 0.21
TOTAL 3.87 3.02 1.79 3.18
DTL, diluted treated leachate; TL, treated leachate; IF, inorganic fertiliser; RL, raw leachate
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Dry biomass weight evaluation
B. rapa L. that received 50%DTL+50%IF presented 2.25 and 1.60 times higher dry weight of leaf biomass than plants treated with the control and 100%IF (Table 6). This phenomenon may be attributed to the synergistic effect of N, which enhanced moisture retention in plants treated with 50%DTL+50%IF plants.42 Statistical comparison showed that leaf dry weights were not significantly different for plants treated with 50%DTL+50%IF and those treated with 25%DTL p<0.05, which could be because of equal moisture contents and/or evapotranspiration rates during fertigation.43 The dry root biomass of plants treated with 25%DTL was 3.16 and 1.70 times higher than that of plants treated with the control and 100%IF, respectively (Table 6). Initial low biomass yield, moisture content in the control and optimum growth nutrient requirements may be the implicating and/or limiting factors. Dry stem biomass was 5.48 times
lower in control plants (0.25±0.050 g) than that in plants with 100%RL (1.37±0.176 g, p<0.05). The zero N supplementation during fertigation hindered the yield and subsequent biomass.
Heavy metal analysis in soil and plants, pH impact and N dynamics
The overall concentrations of heavy metals present in the soil prior to fertigation were lower than the detection and maximum permissible limits.44,45 Application of leachate generally altered the physicochemical characteristics of the soil and the heavy metal uptake by vegetables.46,6 A comparison between edible (plant) parts, namely, shoots collected 1 cm above the soil surface, showed that plants treated with 50%DTL and 100%IF and market samples of B. rapa L. (as control), contained zero Cd (Tables 7 and 8), while the permissible limit for Cd is 0.2 mg/kg.47,48 Table 6: Dry leaf, root and stem weights of Brassica rapa L. after 56 days
Treatments Leaf (dry wt.) g Root (dry wt.) g Stem (dry wt.) g
100%RL 2.80±0.427zcd 91±0.553zcdfgh 1.37±0.176zcdefgh
100%TL 2.55±0.247zd 3.30±0.500zcdfgh 1.21±0.110zdefh
75%DTL 2.22±0.475x 1.18±0.457x 1.00±0.474zdefh
50%DTL 1.95±0.377x 0.95±0.182x 0.33±0.11x
25%DTL 3.60±0.304zybcdfh 3.60±0.654zycdfgh 021±0.015x
12.5%DTL 2.67±0.301zd 1.25±0.050x 0.58±0.104e
50%DTL+50%IF 3.95±0.050zybcdfh 1.33±0.104x 0.93±0.104zdefh
100%IF 2.48±0.225z 2.11±0.202zcdfg 0.33±0.029x
dH2O 1.75±0.180 1.14±0.051 0.25±0.050
TOTAL 23.97±2.586 17.77±2.753 6.21±1.178
Level of significance: p>0.05 at F=15.876 p>0.05 at F=24.543 p>0.05 at F=17.25
DTL, diluted treated leachate: TL, treated leachate: IF, inorganic fertiliser: RL, raw leachate
Letters indicate statistical significance between different treatment levels using analysis of variance (ANOVA) version SPSS 17.0.
Table 7: Metal content comparisons of 50% diluted treated leachate (DTL) with 100% IF treatment levels with both (water and market) controls for Brassica.
rapa L. after 56 days
Metals 50%DTL (mg/kg) 100%IF (mg/kg) dH2O Control (mg/kg) Market Vegetable Control
(mg/kg)
Shoot Root Soil Shoot Root Soil Shoot Roots Soil Shoot Root Soil
K 42.84±4.1 3.16±4.3 1.21±0.0 62.89±3.9 3.27±2.0 1.31±0.2 0.95±0.3 1.84±0.2 1.31±0.0 12.46±2.2 11.24±3.1 – Ca 18.35±2.9 14.06±3.7 5.32±0.1 41.99±4.0 2.56±0.9 6.06±0.2 0.22±0.1 1.26±0.1 5.7±0.4 6.6±1.2 3.43±0.8 – Mg 5.65±0.03 4.85±0.5 0.92±0.0 5.74±1.4 4.96±1.2 1.83±0.1 3.06±0.8 5.73±0.5 2.04±0.0 1.78±0.2 10.85±0.2 – Na 41±02±3.2 30.40±4.4 5.29±0.1 15.53±1.4 2.93±1.9 6.03±0.1 2.16±1.0 2.93±0.2 2.97±0.0 5.26±2.1 3.65±0.1 –
Pb 0.09±0.12 0.41±0.3 0.00±0.0 0.07±0.10 0.19±0.1 0.00±0.0 0.00 0.00 0.00 0.00 0.00 –
Cd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 –
Se 1.36±0.10 1.97±0.2 1.50±0.0 0.12±1.2 1.24±0.0 1.35±0.3 0.04±0.0 0.04±0.0 1.23±0.0 1.15±0.0 1.09±0.0 – Al 20.05±3.5 16.11±2.7 3.12±0.0 0.83±2.3 3.49±1.6 2.62±0.0 4.55±0.7 4.80±0.8 5.69±0.2 4.55±0.0 4.80±0.8 – Mn 0.43±0.22 0.49±0.1 0.07±0.0 0.18±0.4 0.43±0.0 0.13±0.0 0.17±0.1 0.08±0.0 0.15±0.0 1.22±0.2 0.20±0.0 – Cu 0.16±0.02 0.25±0.0 0.07±0.01 0.01±0.36 0.20±0.2 0.02±0.0 0.04±0.0 0.03±0.0 0.09±0.0 0.10±0.0 0.07±0.0 – Zn 0.82±0.47 1.16±0.1 0.32±0.00 0.23±1.56 1.29±0.4 0.40±0.0 0.40±0.1 0.33±0.1 1.00±0.0 0.90±0.1 0.34±0.1 – Fe 8.82±4.15 9.98±0.4 0.37±0.00 0.91±2.55 2.51±2.8 0.28±0.0 4.34±0.4 4.00±0.4 0.93±0.0 4.11±1.1 6.50±1.2 –
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Table 8: Food and Agricultural Organization of the United Nations/World Health Organization (FAO /WHO) heavy metal permissible limits in vegetables
Metals FAO/WHO (mg/kg)
K N.A
Ca 75
Mg N.A.
Na N.A.
Pb 3
Cd 0.2
Se N.A.
Al N.A.
Mn 0.2
Cu 40
Zn 60
Fe N.A.
As 1.0
N.A, not available
Source: FAO/WHO (Codex Alimentarius47,48).
Traces of Pb ranging from 0.07–0.09 mg/kg were detected in B. rapa L.
edible parts treated with 50%DTL and 100%IF, whereas zero Pb content was observed in the market sample. Nevertheless, Pb concentration was still lower than the maximum permissible concentration of 3 mg/kg.
Moreover, arsenic content was lower than the maximum permissible concentration limit of 1.0 mg/kg, as proposed by the Food Quality and Standard Control Division, Ministry of Health Malaysia49 for heavy metals under the Malaysia Standard.
Large amounts of K, Ca, Mg, Na, Al, and Fe accumulated in the edible parts of B.rapa L. treated with 50%DTL were compared with those in the market sample, but the concentration of these elements was still within the acceptable range. This result is in agreement with a previous study by Liu et al.50 that reported increased levels of metals in edible parts of food crops that were continuously irrigated with wastewater. Conventional wastewater treatment processes concentrate sludge fractions with accumulated heavy metals, thereby producing pure water with minimal metal concentrations.51 The results of this study were obtained solely from a greenhouse experiment and should therefore be verified with field studies before being advocated to farmers. Long-term fertigation with landfill leachates may improve soil health conditions because of the fertiliser effect and provide economic benefits to farmers. However, this process may present certain limitations, particularly in terms of soil salinisation, N oversupply, heavy metal leaching to ground water and food chain contamination.51
The soil pH was 6.02±0.01 before fertigation began and then decreased to 6.0±0.001 and 6.01±0.002 for the undiluted (100%) and diluted (12.5–75%) treatments, respectively. Cation exchange capacity, clay fraction, and soil organic matter were implicated in influencing the buffering system in acid soils, as reported by Jiang et al.52 N incorporation from leachate was assumed to be responsible for the decrease in pH.
A field inquiry performed by Van Breemen et al.53 and Huang et al.54 indicated that 500 kg N/haper yearas urea is commonly applied the for production of B. rapa L. Based on this urea content, which is equivalent to 35.7 kmol N/ha per year, the estimated N input to 5 kg of Mollisol in a poly bag is 0.14 kmol N kg/ha per year. As the N concentration of undiluted leachate (100%) was 0.09% and that of diluted leachate (12.5–75%) ranged from 0.012–0.068%, the increase in H+ protons(pro) caused bynitrification (Ni), amounted to 0.013 and 0.002 to 0.009 kmol kg/haper year respectively, as calculated using Equation 255:
H+(pro) (Ni) = 0.14 x % N(Leachate) kmol kg/haper year Equation 2 Thus, N use efficiency from harvested plants in the undiluted (100%) and diluted (12.5–75%) leachate treatments amounted to 40% and 60%, respectively. Hence, H+ proton deposition(dep) in soil calculated using Equation 3,56 was 0.06 and 0.08 kmol kg/ha per year for the undiluted diluted leachates, respectively.
H+(dep) (uptake) = 0.14 x %N(utilised) Equation 3
The duration (1/T) for a unit decrease in soil pH on account of leachate treatments was estimated with Equation 4:
1/T = H+(dep) x Bulk Density [BD = 0.02: mass (5 kg)/vol. (250mL)]
Equation 4 For the undiluted (100%) and diluted (12.5–75%) leachates, the estimated duration was 625 years and 833 years, respectively. The soil acidification rate induced by N fertigation with leachate was approximately 0.01 unit pH/year.
Conditions such as extreme biogeochemical/anthropogenic disruptions in the soil ecosystem may alter and/or decrease soil pH and enhance cation bioavailability and desorption from soil matrices. In cases with rapid changes in soil pH because of fertigation with leachate, application should be terminated and waste composition from the leachate source, treatment facility, and concentrations applied to soil should be re-evaluated.
Generally, B. rapa L. exhibits higher mineral accumulation tendencies at their leaf region. This study revealed that K, Ca, Mg, Na, Al, and Fe were the most dominant minerals present in the plant. Pb and Cd concentrations were lower than the permissible levels.
Conclusion
This study attested that treated landfill leachate, similar to inorganic fertiliser, can be an effective source of nutrients for irrigated B. rapa L.
Leachates can be recycled and utilised as bio-fertiliser for edible and non-edible plants, even for ornamental and timber species. The heavy metal levels in the treated leachate-grown B. rapa L. were lower than the stipulated permissible concentrations based on the FAO/WHO standards.
Therefore, this treatment strategy will reduce the impact of chemical fertilisers on our ecosystem.
Acknowledgements
This research was supported by the IPPP-University of Malaya, Malaysia under grant No. IPPP/PV054/2011A. We thank the Solid Waste Laboratory, Institute of Graduate Studies, University of Malaya and the graduate students who supported this research work.
Authors’ Contributions
F.O.A. was the principal researcher, while the P.A. was the project head and investigator. Both authors collaborated effectively to complete the study.
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